Re: (ET) Replace a lift strap, motor, etc....

["David Roden" <etpost drmm net>]

On 25 Jan 2015 at 20:45, Chris Zach wrote:

>  The RPMs really don't go up much under no load, as the series field
> plus the series armature cancel each other out. 

No, they don't.  A series motor MUST have a load.  Operated at nameplate 
voltage without any load, it will speed up until it flies apart.  

As a series motor's speed increases, the  current it draws decreases.  
Since 
current is the same everywhere in a series circuit, the field current 
falls, 
and so does its magnetic flux.  This makes the motor speed up even more.  
And that, folks, is how runaway happens.  If you're lucky, the motor's 
case 
will keep the shrapnel from ripping holes in YOU.

For testing an unloaded series motor, you can only apply a fraction (maybe 
10%) of the rated voltage without danger of runaway.

I suspect that the reason an unloaded ET lift motor doesn't overspeed is 
that the worm gear wastes a bunch of power, ensuring that the motor always 
has at least some load.


David Roden - Akron, Ohio, USA

= = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = 
Note: mail sent to the "etpost" address will not reach me.  To send 
me a private message, please use the address shown at the bottom
of this page : http://www.evdl.org/help/
= = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = =